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In a new study, guys who did high intensity interval training (HIIT) for just 13 minutes burned more calories per minute on average and upped their maximal oxygen consumption, or VO2 max (which governs aerobic endurance), by 12.5% more than those who did normal cardio for 40 minutes—that is, more than twice as long. “We suggest three [high-intensity] three-minute sets with a two-minute, middle-intensity active rest between sets,” says study co-author Tomoaki Matsuo, Ph.D.